1. Field of the Invention
This invention relates to a cultivating system for performing cultivating work such as plow work using a tractor and a plowing machine while controlling and keeping a plow depth to a desired value irrespective or variation of cultivation environments such as unevenness of the surface of soil, variation of soil type, variation in hardness of soil, etc.
2. Description of Related Art
It has been generally known that subsoil breaking work is indispensable to make such an improvement of soil that water retentivity is provided not only to a sole pan which is located below a tilth layer, but also to a subsoil layer which is located below the sole pan and has a water-impermeable property through breaking work of the sole pan and the subsoil layer in a field. For growing of crops, It is ideal that the tilth layer has a water retentivity for keeping a proper amount of water therein and it has a sufficient thickness. When the tilth layer is slightly dried and subsoil contains water, roots of crops can extend deeply into soil and absorb water, so that these crops can avoid suffering from a drought even when rain is insufficient.
For example in a case where a dry field such as a plowed field, a farm or the like is required to be formed in a wet field such as a paddy field, the dry field must be formed to have a capability of surely excluding water in its tilth layer because crops to be grown in the plowed field are liable to surfer greater damage than those in paddy field when they are immersed in water.
For a paddy field, particularly a paddy field having heavy clay soil, its subsoil is liable to be consolidated due to pressing and puddling or soil by a large tractor, so that soil properties such as gas permeability, water permeability, water retentivity, etc. are degraded. In addition, the tilth layer becomes gradually shallower by a rotary tillage or cultivating work which is repeated in every year, and thus a water-impermeable sole pan is formed below the tilth layer.
Even when a culvert is provided at the lower side of the sole pan, in a large amount of rain, wter is kept in the tilth layer to become an excessively wet state, that is, in a muddy state. Conversely, When there is no rain for a long time, the sole pan prevents water-absorption of a lower layer portion below the tilth layer, so that the tilth layer is kept in a dry state and suffers drought damage.
Accordingly, when a dry field such as a plowed field is formed in a wet field such as a paddy field, it is required that a sole pan of the dry field, which deteriorates water permeability and water retentivity, is broken, and at the same time a subsoil layer below the sole pan is kept in a soft state to improve gas permeability and water permeability of soil, and that a cultivation depth is set to a depth suitable for growing of crops in the plowed field, for example, 20 to 30 cm.
In view of the foregoing, the following methods have been hitherto adopted. That is, in one method, the sole pan is broken and subsoil is made soft by a subsoil breaking machine, and then the tilth layer is plowed and reversed by a plowing machine. In the other method, both of the tilth layer and the sole pan are simultaneously plowed and reversed, and the subsoil layer is made soft.
In a case where a cultivation environment is not homogeneous in a cultivating or plowing direction in a field, for example hardness of soil is not constant in the direction of cultivating work and thus hard and soft areas alternately exist along the cultivation direction in the field, a bottom position of a shear varies in accordance with the variation of hardness of soil. For example, the shear of plow is lifted up (floated) at a hard area because the friction resistance between the shear and soil is large at the hard area. On the other hand, the shear moves deeply into soil at a soft area because the Friction resistance is small at soft area. Accordingly, an operator on a tractor must control the position of a chisel (plow) through a manual lift operation wherever the hardness of soil varies.
Further, in a case where soil of the field has an uneven surface in the plowing direction, time cultivating machine such as a plow machine is shifted upwardly or downwardly whenever wheels of the tractor pass over a recess or projecting portion, and thus the plowing depth is varied in accordance with unevenness of the soil surface. For example, when the tractor passes over an uneven area, the attitude of the tractor is varied to a backwardly-tilted or forwardly-tilted state. In the former state, the plow machine is forwardly tilted and thus the shear of a bottom plow trends to deeply enter into soil. In this case, a load imposed on the plow is increased. On the other hand, in the latter state, the plow machine is backwardly tilted and thus the shear trends to be floated, and thus the sole pan and the subsoil layer cannot be sufficiently broken.
As described above, in cultivating for soil using a well-known tractor and cultivating machine such as a plow, subsoiler or the like, the cultivating (plowing) depth is required to be kept to a desired depth at all times. In order to meet this requirement, in addition to the manual control of the position of the chisel as described above, it has been also proposed that the tractor is provided with a draft function or that a lift mechanism is actuated while measuring the height of the plow from the ground.
Specifically, in one method, the cultivating machine is upwardly or downwardly moved while detecting an upward or downward motion of a gage wheel which moves in contact with the ground, in other words, while detecting an angle between a wheel arm got supporting the gage wheel and a frame of the cultivating machine. In another method, ultrasonic wave or light is emitted to the ground and a current cultivation or plow depth is measured by detecting ultrasonic wave or light which is reflected from the ground. The cultivating machine is upwardly downwardly moved on the basis of the measurement result. The former method is representatively disclosed in Japanese Utility Mode Publication (KOKOKU) No. 5-1221.
In such a cultivating system, a tractor and a cultivating machine such as a plow are linked to each other through a three-point link (hitch) mechanism. The linkage between tractor and the cultivating roaching is regarded as a rigid joint, and thus the joined body of the tractor and the cultivating machine must be treated as one rigid body. Therefore, when an excessive load which exceeds the tractive capacity of a tractor is imposed on the tractor, the following troubles occur .in the conventional cultivating system. That is, when such an excessive load is imposed on the tractor, the tractor is kept in such a state that front wheels of the tractor are floated while rear wheels thereof are contacted with the ground. Therefore, the tractor loses its tractive force, and it is impossible for the tractor to continue cultivating work. On the other hand, when the tractor forcedly continues the cultivating work under such an excessive load in a state where the front wheels thereof are not floated, the cultivating machine (plow machine) may be damaged. In order to prevent such troubles, a tractor is designed to a heavy and rigid body to increase its weight because the tractive capacity of the tractor is proportional to its weight. In addition, with respect to the cultivating machine, it is also designed to a heavy and rigid body, and these heavy and rigid tractor and cultivating machine are linked to each other. As described above, the "excessive-load trouble" as described above can be overcome by increasing the weight of the tractor and the cultivating machine (cultivating system), and thus by increasing the tractive force of the cultivating system.
Those tractors which are used for cultivating works in large-scale fields, for example, in U.S.A., Europe, etc., are generally designed in a relatively large size with a large power. Therefore, the large-scale and large power tractors and cultivating machines suitable for these tractors have been widely used in such countries. However, those tractors which are used for cultivating works in small-scale fields, for example, in Japan, are generally designed in a relatively small size with a small power because such compact tractors are capable of making a small sharp turn in relatively small-scale fields. Therefore, the small-scale and small power tractors and cultivating machines suitable for these tractors have been widely used in Japan.
If these tractors and the cultivating machines which are suitably used for small-scale fields are applied to geographically complicated fields, for example, a field whose soil surface is uneven in a cultivating direction, a field having various soil types (variation in hardness of soil) in a cultivating direction, etc., cultivating work cannot be efficiently performed for the teasotis as described above. Therefore, the large-scale and large-power tractors have been hitherto required to be used in such complicated fields.